The present invention relates to vibrometry and vibrometers. Vibrometers measure the vibrations of an object in response an impinging beam (usually a laser beam) for the purpose of measuring the vibrations which result therefrom. Vibrometry and vibrometers can be used for non-destructive testing of products and can also be used in the military field for landmine detection, target identification, lethality assessment, etc.
In the field of vibrometry, the prior art includes Fabry Perot inferometers which suffer from being bulky, expensive, and offer poor resolution at low frequencies for a reasonable length device. The prior art also includes Phase Conjugate Mirrors (PCMs), which tend to be difficult to align because they tend to require diffraction-limited plane waves with precision alignment. Thus, typical PCMs, besides being difficult in terms of alignment, would not be expected to stand up well in the field where the military might be using the device. The prior art also includes self-pumped PCMs (SP-PCMs), which only use a single beam to function, the beam being the same beam whose wavefront-reversed replica is sought. This type of SP-PCM is sometimes referred to as a xe2x80x9ccat conjugator.xe2x80x9d Such SP-PCMs function over a limited Field Of View (FOV), are typically slow to respond (since the incident, scattered beam may be very low in power at the device), and, moreover, typically possess phase-conjugate reflectivities less than unity (typically, in the 25% to 35% range). As such, the prior art SP-PCMs have a number of disadvantages which become particularity evident when the object being tested has a low reflectivity to light.
The prior art also includes double-pumped PCMs (DP-PCMs), but these devices have a problem of having to match the intensity of both beams impinging the DP-PCM. The prior art also includes two-wave mixers, but these devices tend to be FOV limited. The prior art also includes single-pixel interferometers, but these devices suffer from poor light gathering efficiency, a limited FOV, dropouts and moreover rely on fortuitous speckle, which implies a long dwell time in order for the device to function. The prior art also includes photo-emf devices, but such devices are not shot-noise limited, have a limited FOV and also have a limited bandwidth. The prior art further includes adaptive optical mirrors, but such devices tend to have a limited number of pixels, are complex and have bulky electronics associated therewith.
Many objects whose vibratory signature is sought have rough surfaces and thus tend to scatter the incident probe beam into many speckles. The scatter light speckles can find their way back into prior art vibrometers which will tend to degrade the performance of those vibrometers. In order to avoid this sort of degradation, prior art vibrometers employ focusing optics to limit the returned light to a single speckle.
The present invention addresses many of the problems associated with prior art vibrometers. The present invention uses a self-pumped xe2x80x9ckittyxe2x80x9d Phase Conjugate Mirror (PCM) with gain. xe2x80x9cKittyxe2x80x9d PCMs are described by J. Feinberg in Opt. Lett. 8, 480 (1983) and also discussed below. This device tends to have a wide FOV (the FOV may well fall in the range of 45xc2x0 to 90xc2x0 or even greater), can generate a phase-conjugate wave with greater power than the incident beam (e.g. gains in the range of 10 to 10,000) and enables the vibrometer with such a device to function even if the surfaces being sampled are not specular or polished, but rather are lossy (i.e. the surfaces being sampled exhibit low reflectivities and the reflected light can include thousands to millions of speckles). Indeed, in the disclosed embodiments, the detector associated with the vibrometer is allowed to xe2x80x9creadxe2x80x9d many or most of those speckles (the more speckles which are detected the better, but at least some speckles are likely lost). Thus, the device can be used to measure vibrations in the earth, composite structures, unpolished metallic or ceramic surfaces, or other highly scattering surfaces. Of course, the device can also be used to measure the vibrations of polished surfaces, but the surfaces whose vibrations are to be measured, need not be polished.
In one aspect, the present invention provides a vibrometer for probing an object at a distance to determine its vibratory signature, the vibrometer comprising: a laser for generating a laser beam for probing the object; a detector for detecting reflections from the object; and a two beam input phase-conjugate mirror located so as to receive a portion of the laser beam produced by the laser and a portion of the laser beam reflected from the object, the two input beam phase-conjugate mirror reflecting received diffuse (multi-speckled) signals from the object back towards the object in a wavefront reversed manner, thereby compensating for wavefront distortions.
In another aspect, the present invention provides an apparatus for improving a lasing vibrometer, the apparatus comprising a two beam input phase-conjugate mirror located so as to receive a portion of a laser beam produced by the vibrometer and a portion of a laser beam reflected from an object whose vibratory signature is sought, the two input beam phase-conjugate mirror reflecting received laser light reflected by the object back towards the object as a wavefront-reversed replica of the received laser light.
In still yet another aspect, the present invention provides a method of obtaining a vibratory signature of an object, the method comprising the steps of impinging the object with a probing laser beam; directing light scattered by the object to a two beam input phase-conjugate mirror; pumping the two beam input phase-conjugate mirror; directing light reflected by a two beam input phase-conjugate mirror back to the object; and directing light from two beam input phase-conjugate mirror which is reflected by the object towards the probing laser beam towards a detector apparatus, with diffraction-limited beam quality and with an optical power level that can exceed that of the incident probe beam that illuminated the object initially.